1. Field of the Invention
The present invention relates to a hydrometallurgical process of nickel oxide ores, more particularly a hydrometallurgical process based on pressure leaching at elevated temperature for recovering nickel from nickel oxide ores, characterized by a simplified and efficient process as a whole, realizing a simplified leaching stage and solid-liquid separation stage, reduced neutralizer consumption and precipitate production in a neutralization stage, and efficient use of recycled water.
2. Description of the Prior Art
Recently, pressure acid leaching at elevated temperature (High Pressure Acid Leach) in the presence of sulfuric acid, has been attracting attention as a hydrometallurgical process of nickel oxide ores by virtue of advanced materials developed to efficiently exhibit corrosion resistance at elevated temperature and pressure. This process is more energy-efficient and advantageous costwise than the pyrometallurgical process, which has been commonly adopted for treating nickel oxide ores, because it is an integrated wet stage involving no dry stage, e.g., reduction or drying. Therefore, various approaches, centered by those for the high-pressure acid leaching stage, e.g., for washing a pregnant liquor, have been proposed to establish the more improved process.
Many of these proposals, however, are mainly aimed at improvement of leaching rate of nickel or cobalt, reduction of operational material requirements and recovery of by-products, and few are related to an overall process of nickel oxide ores involving series of stages up to a washing system.
One of the representative pressure acid leaching processes at elevated temperature for recovering nickel, cobalt or the like from nickel oxide ores, leaches a residue from an atmospheric leaching stage with sulfuric acid at elevated temperature and pressure in an oxidative atmosphere; leaches an oxide ore slurry at atmospheric pressure in an acidic atmosphere by sulfuric acid with the pregnant liquor discharged from the above stage; neutralizes the pregnant liquor discharged from the atmospheric leaching stage; and treats the slurry with an alkali sulfide compound to recover nickel or cobalt in the form of sulfide (disclosed by, e.g., JP-A 6-116660 (Pages 1 and 2)).
The above process first leaches a slurried ore with a pregnant liquor containing sulfuric acid at a high concentration from an autoclave in an atmospheric leaching stage. This process has advantages of reduced load in a neutralization stage, brought from consumption of the excessive acid present in a pregnant liquor of the high-pressure acid leaching stage, and improved rate of recovering nickel from the ore. These advantages are due to use of the atmospheric leaching stage. On the other hand, however, the atmospheric leaching stage causes a disadvantage of increased number of facilities for slurry concentration in a process involving a series of stages, e.g., atmospheric leaching, thickener-aided slurry concentration and pressure leaching at elevated temperature, which deteriorate system simplicity and operability of the overall process.
The leached residue from the atmospheric leaching stage is then treated by the pressure leaching stage at elevated temperature, and the resulting pregnant liquor is treated by a series of stages including solid-liquid separation, neutralization and sulfide precipitation to recover nickel and cobalt. JP-A 6-116660, however, is silent on the detailed operating conditions. The leached residue washing stage, for example, normally needs a large quantity of wash water for sufficiently recovering nickel from the residue, which increases the refining cost when fresh wash water is continuously used. The document, however, is silent on the above disadvantage.
In addition, the neutralization stage normally involves a problem of nickel loss in the neutralization precipitate. This loss is caused by water deposited on the precipitate, and locally increased pH level on the neutralizer to form the hydroxide. Hydroxylated nickel is normally recovered with sulfuric acid excessively remaining in the pregnant liquor. In other words, the precipitate produced in the neutralization stage is recycled back to the leached residue washing stage to recover nickel by leaching with sulfuric acid remaining in the pregnant liquor. This, however, redissolves iron hydroxide present in the precipitate. As a result, it should be removed by hydroxylation of iron once again in the neutralization stage, which increases neutralizer consumption. It is therefore desirable to minimize the precipitate recycled back to the leached residue washing stage. JP-A 6-116660, however, is also silent on abatement of the precipitate produced in the neutralization stage.
Under these situations, the pressure leaching process at elevated temperature of nickel oxide ores has been demanded to be further improved in efficiency and simplicity of the overall process by solving problems involved in each of its stages.